Neuroendocrine (NE) carcinoma and NE differentiation (NED), which occur in 30% of human prostate tumors, are hallmarks of aggressive human prostate cancer. The goal of this application is to extend our exciting findings that reveal a novel mechanism underlying development of prostate NE tumors. This discovery originated from our observation that NE tumors, which form in the TRAMP mouse prostate tumor model, are no longer seen when these mice are crossed with the ubiquitin ligase Siah2 KO mice. This striking result led us to hypothesize that Siah2 activity underlies NE prostate tumor development. Earlier studies supported by this grant identified a role for Siah2 in increasing HIF-11 stability (through ubiquitination and degradation of prolyl hydroxylase1/3 stability). Therefore, observing a loss of NE tumors led us to explore a causative role for Siah2/HIF-1? in NE phenotype and prostate tumor development. Indeed, mice lacking Siah2 exhibited markedly reduced levels of HIF-1? expression. To understand how lower levels of HIF-1? could antagonize NE tumor development we assessed potential HIF-1? cooperation with FoxA2, a transcription factor functioning in prostate tumor development and associated with NE differentiation. Co-expression with FoxA2 markedly increased HIF?'s ability to transactivate HRE-based promoters. Further, we identified a subset of genes regulated by FoxA2/HIF-1? including Hes6, Jmjd1a, Plod2 and Sox9. Of particular significance is that we have confirmed the expression of these select subsets in mouse NE tumors but also in metastatic human prostate adenocarcinoma. Furthermore, co-expression of these genes rescued formation of NE phenotype in human prostate tumor cells expressing Siah2 inhibitor, as well as the ability of these cells to form prostate tumors in mouse xenograft. Overall, these findings led to our hypothesis that Siah2-dependent concerted activity of HIF-1? and FoxA2 plays a central role in formation of NE phenotypes and development of human prostate tumors. We thus propose to identify mechanisms underlying FoxA2 and HIF cooperation and their contribution to NE phenotype and human prostate tumor development. We will therefore (1) determine the role of FoxA2, HIF-1? and Siah in androgen-dependent and -independent human prostate adenocarcinoma development and progression, and in NE phenotype; (2) characterize mechanisms underlying FoxA2 cooperation with HIF-1?; (3) determine the role of select HIF-1?/FoxA2 regulated genes - Hes6, Plod2, Sox9 and Jmjd1a - in prostate tumor development and progression and assess the possible use of these genes as markers for human prostate adenocarcinomas; and (4) identify key domains required for FoxA2 association and cooperation with HIF-1? to determine whether inhibiting FoxA2/HIF-1? cooperation by use of select peptides blocks formation of human NE prostate tumors in mouse xenograft models. Our proposed studies are expected to establish a new regulatory pathway in NE phenotype and prostate tumor development and metastasis.